The goal of this grant is to develop a permanent gene-based treatment for the central nervous system (CNS) component of lysosomal storage disorders with mental retardation. We will investigate the properties of genetically engineered cells as vehicles to transfer lysosomal enzymes into the brain. In the previous grant period, we have demonstrated that: 1) retroviral vectors can express GUSB at high levels in MPS VII cells; 2) the transferred enzyme corrects the degradative defect within diseased cells; 3) the transduced cells export GUSB which is taken up by mutant target cells (cross-correction); and 4) the normal enzyme can correct the characteristic pathology in diseased animals. We have unequivocally demonstrated that gene therapy can work, in principle, for this class of diseases. However, there are still substantial barriers to achieving permanent and complete correction of the widespread lesions in the brain, which is the long-term goal of these studies. The major problems that need to be solved in the next phase of the research are: 1) to achieve sustained expression of the transferred normal gene after the cells are transplanted into the brain; 2) to identify the optimal cell type for correction and transplantation to the CNS in this disease; 3) to determine the extent of diffusion of the corrective enzyme through neural tissue; and 4) to determine the extent of phenotypic correction that can be achieved from a graft that continuously produces normal enzyme. To address these goals we will: 1) develop modified retroviral vectors to maintain long-term expression of GUSB from corrected primary cells transplanted into the brain; and 2) compare fibroblasts, glial cells, and neuronal progenitors for long-term engraftment and delivery of therapeutic levels of GUSB.